Art of tin plating



United States Patent ART or TIN PLATING John Walter Nelson, Hammond, Ind., assignor to Sinclair Refining Company, New York, N. Y., a corporation of Maine No Drawing. Application May 19, 1950, Serial No. 163,077

1 Claim. (Cl. 117-114) My invention relates to the use of tinning oils based upon certain synthetic nr'icrocrystalline wax acids of extremely high molecular weight in hot dip tinning operations.

In making hot dipped tin plate according to conventional practice, sheets of pickled steel are passed by means of rolls through a flux into a bath of molten tin and up through an eighteen-inch layer of hot palm oil, maintained at 450 F., floating on the tin. Three sets of rolls, operating in the oil, smooth and thin the tin coating. The palm oil serves to remove oxides that tend to form on the surface of the tin and also to retard atmospheric corrosion as well as assisting in handling during fabrication. Palm oil has been the only really commercially satisfactory tinning oil heretofore employed. It has, however, several deficiencies. The flash and fire points of palm oil are low enough to create a fire hazard. It is quite volatile at tin pot temperatures and tends to increase in viscosity and polymerize with time at tin pot operating temperatures. In addition, a primary disadvantage is its foreign source, and the strategic and financial problems which thereby arise.

In my copending application, Serial No. 157,177, filed April 20, 1950, now Patent No. 2,610,974, there are disclosed certain new microcrystalline wax acids of synthetic origin. These acids have an unusually high molecular weight, containing more than eighteen carbon atoms per molecule and, usually, in excess of twentyfour carbon atoms per molecule, and ranging up to forty and more. The acids are prepared by oxidizing microcrystalline waxes having 34 to 55 carbon atoms per molecule in the presence of a stoichiometric excess of oxygen and about 0.1 to 4.0 per cent by weight of an oxidation catalyst at a temperature in excess of about 100 C. for a period of time sufi'icient to effect substantially complete conversion. The pure C1a+ wax acids are then separated from the reaction mixture, as by washing and distillation. The acids are essential ly monocarboxylic andare characterized by extreme insolubility in water. They have low melting points and are not readily volatile even at high temperatures.

I have found that tinning oils based upon the synthetic C18+ microcrystalline wax acids function. very satisfactorily in making hot dipped tin plate. The flash and fire points of the wax acids are higher than those of palm oil. The acids are not as volatile hence the loss in operation is lower. The C1a+ acids do not increase in viscosity unduly on prolonged heating, an advantage since palm oil is discarded when its viscosity rises above a specified control point. The acid material is easily cleared from the freshly tinned surface in the conventional manner. The wax acids have good dewetting properties. That is, their activity in removing tin oxides that may form on the surface of the sheet is highly satisfactory. In addition, the use of the acids in tinning has the special advantage that readily available low-cost materials of petroleum origin areprovided in place of the conventional and expensive palm oil which must be imported.

According to my invention the conventional hot dip tinning process is carried out with tinning oils based upon the C1s+ synthetic microcrys'talline wax acids. I have found that the wax acids themselves are especially useful for such tinning operations. The acids are gen erally characterized by favorable flash and fire points, good stability at operating conditions and excellent activity. In particular, the heavier acids are preferable since their flash and fire points are higher. That is, acid mixtures having saponification numbers of about 130 and less, or an average of about twenty-eight car bon atoms per molecule and more, are especially useful considering flash and fire points as well as lower volatility and favorable activity.

In addition, dispersions of the (118+ microcrystalline wax acids in oils also provide good tinning oils, such as 50-50 volumetric blends with an oil having an S. U. V. of to 200 seconds at 100 F. Advantageously, dispersions of a heavier acid mixture with a mineral oil of high flash and fire points form tinning oil products of correspondingly high flash and fire points.

Derivatives of the acids may be formed into good tinning oils, such as a glyceride or ester. In particular, esters of the lighter acids may be used which increase the molecular weight and thereby raise the flash and fire points and lower volatility, yet which have good activity. The corresponding alkyl esters may be formed, such as the butyl ester. In addition, the glyceridic or ester derivatives may be dispersed in a suitable mineral oil to likewise provide highly useful tinning oils.

Hot dip tinning with tinning oils according to my invention is particularly satisfactory because of the activity and good dewetting properties of the C1a+ microcrystalline wax acid materials. Favorable tinning oils must necessarily have good dewetting properties. That is, the oil must be active in removing the objectionable tin oxides as they form on the surface of the tin. From the theoretical view point an active, favorable tinning oil has a strong physical afilnity for tin oxide and a powerful dispersing action in it. The combination of the cohesive force in the main body oil, the dispersibility of the oxide film in the oil and the low friction between oxide and molten tin results in the film being pulled under the body of oil and dispersed. The layer of oil drains from the sheet until it is sufliciently attenuated for oxygen to diffuse through it to the molten tin. Once the oil break has started it proceeds rapidly, for instantaneous oxidation of the molten tin exposed to air continually forces a negative contact angle between the tin and the oil. The Petri dish test may be used for evaluating the activity and dewetting properties of tinning oils. The test is suggested in Palm Oil Substitutes for Hot Dip Tinning, in a preprint of a paper read before the General Meeting of the American Iron and Steel Institute at New York, May 2526, 1949, by W. R. Johnson, L. C. Kinney and John M. Parks, of the Armour Research Foundation, Chicago, Illinois. According to this test, a Petri dish is partly filled with molten tin, and an oil is added in just sufficient quantity to cover the surface. Inactive oils-defined as oils which remain in a continuous quiet film over the molten tin-wet the'tin and do not disperse or remove any tin oxide which may be present. Active oils break and expose an island of bright molten tin; they are in continuous motion, removing any oxide from the tin surface as fast as it forms. The oil remains in motion until it has become depleted, at which time it creeps over the surface of the tin and behaves like an inactive oil. The C1s+ acids behave as active oils and readily remove any tin oxide that may form. Further, their their derivatives have satisfactory flash and fire points.

The flash and fire points are high enough to eliminate any fire hazard, that is, they are sufiiciently above tin pot operating temperatures 450 F., to minimize fire danger. Generally, the flash and fire points are at least 465 F.,

and usually over 500 F., and depend largely upon the molecular weight of the acid fraction.

'The Crs-I- microcrystalline wax acid materials are not particularly volatile hence loss in operations is low. It has been estimated that about one-third of the palm oil consumption is due to volatile losses; the (315+ wax acid loss in operations is considerably lower. In addition, the Cur-lacids do not increase in viscosity unduly on prolonged heating,jan advantage over palm oil, which does and hence must be discarded when its viscosityrises above a specified control point.

The microcrystalline wax acid material is easily cleaned from the freshly tinned surface in the conventional manner. That is, in normal tin pot operations, a quantity of the tinning oil'is dragged by the sheet out of the bath into the catcher rolls. rolls by condensation of the volatile products from the bath. The sheet emerging from the catcher rolls, therefore, carries a greater quantity of oil than can be tolerated. In most cases, this is largely removed in an alkaline wet washer by a combination of emulsification and mechanical scrubbing. The remaining oil is partially removed by polishing the sheet in a branner. The use of the wax acid materials also has the advantage in that,

generally, ordinary cleaning procedures may be followed without extensive redesign of the cleaning equipment.

It appears from the experiments of Messrs. Johnson, Kinney and Parks, as disclosed in their paper referred to above, that the carboxyl group is essential to a satisfactory tinning oil. 'In particular, the availability of the carboxyl group determines the activity of the oil. However, all of the fatty acids, which contain this group, are clearly not satisfactory. In fact, to the best of my knowledge, very few good tinning oils of any description are presently known. Poor all around operating characteristics, such as high volatility and viscosity, are the prin- V cipal reasons why very few substances meet tinning requirements. Onthe other hand, it is my belief that the particular structure 'of the wax acids, which are especially characterized by the presence-of the carboxyl group and the long carbon chain of the Wax acid molecule, is what largely accounts for their utility as tinning oils, although I have not been able to ascertain positively the exact molecular configuration of the acids.

The wax acids I use in hot dip tinning operations are,

as I have stated, of unusually high molecular weight, containing more than'eighteen carbon atoms per molecule and usually in excess of 24 carbon atoms per molecule and ranging up to forty and more. They are essentially monocarboxylic, have saponification numbers of about 200 or less and are characterized by extreme insolubility in water. Besides the carboxylic group,'the acids may contain other groups such as lactonic groups or inner esters and the hydroxy, 'ketone, ether, aldehyde, ethylene and acyl groups. The acids have low ionization constants (high Ks. p.). They are insoluble in cold alcohol, although will dissolve in hot acetone or benzene. The acids have a relatively low melting point and, in appearance, are white to amber in color and are hard and smooth to the touch.

The synthetic wax acids can be prepared by oxidizing microcrystalline waves having '34 to carbon atoms per molecule in the presence of a stoichiometric excess of oxygen and about 0.1 to 4.0 per cent by weight of an oxidation catalyst at a temperaturein excess of about 100 C. for 'a'period of time sufficientto eflfectsu'bstantially Additional oil accumulates on the complete conversion of the wax to acids. The wax acids having more than 18 carbonatoms per molecule are then separated from the reacted mixture, as by distillation. For example, a microcrystalline wax derived from a Texas crude and containing 34to 55 'carbon atoms per molecule is oxidized with air in the presence of potas sium permanganate. The reaction is carried out at about C. to C. with 150 to 225 liters of air per kilogram of wax per hour using about 1.0 to 2.0 per cent by weight of potassium permanganate. The reaction is continued until conversion is essentially complete, for. instance, until the reaction mixture has a saponification number of at least about 100, and usually, 200 to 500. The high molecular weight acids, higher than C18, are then separated from the reaction mixture which contains certain small amounts of organic and inorganic materials. This may be accomplished by washing with water and/ or an inorganic acid for the removal of inorganic materials such as catalysts, and then subjecting the mixture to distillation in a flash still under reduced pressure for elimination of any lower acids and other organic substances.

In the preparation of the high molecular weight wax acids, microcrystalline waxes containing about 34 to 55 carbon atoms per molecule are oxidized. The Car to C55 microcrystalline waxes are derived from'higher boiling petroleum distillates and residues such as lubricating oil fractions. The waxes appear to be largely composed of molecules especially characterized by slight branchings in the carbon chain. This structure may be contrasted. to crystalline wax molecules which are essentially straight chain. Generally, the branching in the microcrystalline wax molecule is at random along the chain, each branch appearing to contain an average of about three carbon atoms. The microcrystalline wax to be oxidized may be composed of molecules containing very similar or identical numbers of carbon atoms. Generally, however, the wax will be made up of mixtures of molecules over the range of 34 to 55 carbon atoms as well as having molecules of varying structure. The waxes maybe oxidized in the pure or impure state, although elimination of contaminating substances prior to reaction tends toward better product quality. For instance, a C34 to C55 microcrystalline wax obtained from a Texas crude may be purified for reaction by contact at elevated temperatures with aluminum chloride for a short period of time in the usual procedure well known in the art.

The reaction is carried out in the presence of 0.1 to 4.0 per cent by weight on the wax of an oxidation catalyst. Satisfactory catalysts include those dispersable in microcrystalline wax such as manganese salts, ammonium vanadate and potassium permanganate. The use of potassium permanganate, present in amount of about 0.1 to 1.25 per cent by weight on the wax, is particularly advantageous as respects a shorter reaction period and improved product quality. "In any event, less than about 0.1 per cent of the catalyst results in inordinately prolonged oxidation periods while amonnts greater than about 4 per cent tend to oxidation products heterogeneous, inconsistent and stringy in appearance and poor in color. Oxidation catalyst promoters or sensitizers may be employed to accelerate the reaction rate even more. For example, sodium carbonate, manganese palmitate or other manganese salts, may be added in small amounts as-accelerators, .for instance, in amounts generally equal to or less than the quantity of the oxidation catalyst employed. The oxidation catalyst is advantageously added to the microcrystalline wax prior to commencement of the oxidation. In addition, it is advantageous to add the catalyst to .the wax in aqueous solution and to remove the solvent :by evaporation. For instance, potassium permanganate may be'added as a 15-20 per centby weight solution. The water is removed prior to reaction by applying heat, say by heating the mixture to -150? C., or air or oxygen may be added and the solvent water removed in the course of the reaction. Additional catalyst may be added later, that is, during the reaction tostep up the oxidation rate.

By adding to the reaction mixture as a seed" acid composition derived from a prior run, reaction time may be reduced as much as 50 per cent and is usually at least to per cent less, without any sacrifice in product quality or in reaction yield, over the use of the catalyst alone under similar conditions of reaction. For example, a microcrystalline wax derived from a Texas crude and containing 34 to 55 carbon atoms per molecule is oxidized in the presence of potassium permanganate and 0.1 to 1.25 per cent by weight of seed having a saponification number of about 200 to 300. The reaction is carried out at about 110 C. with 150 to 225 liters of air per kilogram of wax per hour using 1.0 to 2.0 per cent by weight of potassium permanganate. The reaction is continued until conversion is essentially complete, for instance, until the reaction mixture has a saponification number of at least 100, and usually, 200 to 400, and the higher acids are then separated out.

About 0.1 to 4.0 per cent by weight on the wax of the acid composition prepared in a prior run is added. Although the seed may be added before or after commencement of the oxidation reaction or before or after the addition of catalyst, a highly favorable reaction rate consistent with good product quality and yield is obtained by first adding the seed to the wax, then adding the catalyst in aqueous solution and commencing oxidation. After the catalyst is added, the oxidizing gas may be added at reaction conditions. If the catalyst is added in aqueous solution, the solvent water may be removed by evaporation before reaction, as by heating to 145 to 150 C., if desired. Also, as is the case with the catalyst, additional seed may be added during the course of the reaction to step up the oxidation rate. In any event, particularly advantageous reaction rates are obtained when about 1.0 to 2.0 per cent by weight of catalyst is employed and a similar amount of seed. The reaction will not go by adding the seed alone, that is, without at the same time employing the oxidation catalyst. The seed has a saponification number in the range of about 100 to 500, contains in substantial amount wax acid molecules having upwards of eighteen carbon atoms, and has low solubility in water.

The wax acids are formed by oxidation of the reaction mixture in the presence of oxygen, either in pure form or in admixture with inert diluents, say as air. The oxygen is added in at least the stoichiometric amountfor a period of time sulficient to effect complete conversion. Advantageously, the oxygen is used in considerable excess of the stoichiometric quantity which reduces time yet results in a very favorable product. It is preferable to add oxygen, considered as substantially pure oxygen, in amounts in the range approximating to 50 liters per kilogram of wax per hour. An amount of about 35 to liters per hour of oxygen per kilogram of wax is particularly advantageous. In any event, amounts less than about 30 liters per hour of oxygen per kilogram of wax tend to unattractively long reaction periods while excessive quantities, i. e., over liters per hour, are not necessary and are wasteful. The use of pure oxygen or diluted oxygen such as air does not noticeably affect product quality, although a higher oxygen concentration does improve reaction time. Good dispersion of the oxygen into the mixture undergoing reaction is necessary for minimum reaction periods. For instance, oxygen contact and dispersion may be improved by introducing the oxygen into the reaction mass and by constant agitation of this mass during reaction.

Considerable latitude is afforded in oxidation temperature, although the thermal environment should be in excess of about 100 C. for the period of the reaction. Temperatures in the range approximating 100 to 150 C. are preferred. Oxidation temperatures between about 110 to C. afford particularly favorable results, with a minimum of side product and carbon oxide formation and with maximum oxygen absorption. The reaction vessel may be cooled when necessary to maintain the desired temperature range since the reaction after commencement is exothermic in nature.

The reacted mixture is oxidized until the C34 to C55 microcrystalline wax has been completely converted into essentially acids. During the course of the reaction, water and volatile acidic matter are given off in small quantities. Generally, the degree of conversion is determined by the saponification number and the length of time required for complete conversion depends in large measure upon the quantity of oxygen available to the wax undergoing reaction and the accompanying thermal environment. However, the catalyst and seed employed, their proportions and even the exact nature of the wax appear to figure in the reaction rate also. Usually, the microcrystalline wax is oxidized until the reaction mixture has a saponification number of at least 100, and advantageously to saponification numbers of 200 to 300 or more. Saponification numbers of the solid reaction product as high as 300 to 400 are not uncommon and indicate a high degree of or complete conversion as well as a greater degree of cleavage. However, the prolonged period of oxidation is generally at least 30 to 40 hours and reaction time as long as 200 to 300 hours are encountered.

' After substantially complete conversion has been effected, the reaction mass essentially comprises a mixture of wax acids containing a substantial portion of monocarboxylic acids having more than 18 carbon atoms per molecule. The mixture also contains certain small quantities of other organic and inorganic matter such as unreacted wax, lower molecular weight acids and catalyst material. The C13 plus acids may be obtained in pure form by washing the solid mixture free of inorganic materials with Water and/or an inorganic acid, such as hydrochloric acid, and then distilling the mixture to separate out the higher acids. For example, the acid mixture is first washed by adding water and hydrochloric acid. The resulting mass is stirred and permitted to settle. The acidwater layer which separates out is removed. The product may be washed again as with water alone, the water removed after another settling period and the product blown with air to evaporate any remaining water. The washed product is then distilled to remove the substantially pure Crs plus wax acids. This may be accomplished by flash distillation in the presence of steam or under high vacuum, or by molecular distillation, in the usual manner. lor instance, employing flash distillation, the charge stock is preheated and distilled at elevated temperatures under low pressure, advantageously as low as practicable, e. g., 1.5 to 3 millimeters of mercury. By distilling at temperatures in the range of ISO-300 C. the lower acids in the reaction mixture are taken off first. By then flash distilling over about 300 C., and in regulated increasing increments of distillation temperature, successively higher molecular weight fractions over C18 are taken 01f as desired. For instance, the higher molecular weight acids may be separated into a number of fractions, such as into a lower fraction containing Cm to C23 wax acids, having saponification numbers in the range of about 195 to an intermediate fraction containing C24 to C34 acids, having saponification numbers in the range of 154 to 110 and a high fraction containing C35 and higher acids, having saponification number of about 109 and lower.

In place of the distillation for separating the pure C1s+ microcrystalline wax acids from the crude reaction product, the pure acids may be recovered by extracting the reaction product with a selective solvent from the class consisting of the saturated hydrocarbons containing three to twelve carbon atoms per molecule. Propane, butane, iso-pentane and normal pentane are exemplary of such solvents. This improvement is disclosed in my copending application Serial No. 169,215, filed June 20, 1950.

V The following examples are intended to illustrate the utility of specific wax acid materials for hot dip tinning operations.

" Examplel A C1s+ microcrystalline wax acid mixture, having a saponificationnumber of 60, an acid number of 41 and an iodine number of 12, was obtained by oxidizing to complete conversion C34 to C55 microcrystall-ine wax, washing the reaction product with hydrochloric acid and water and flash distilling the washed product for a heavy overhead. The heavy overhead acid product had a petrolatum melting point of 14l F., a flash point of 465 F. and a fire point of 495 F.

The wax acid was tested for activity according to the Petri dish test, as described in the paper of Messrs. Johnson, Kinney, and Parks, referred to above. The results showed that the acid material was active and readily removed any oxide from the tin surface as fast as it for-med. I I

T r Example II I A Cia+ microcrystalline wax acid mixture, having a saponification number of 82, an acid number of 59 and an iodine number'of 12, was obtained by oxidizing to complete conversion C34 to C55 microcrystalline wax, washing the reaction product with hydrochloric acid and water and flash distilling the washed'product for a heavy overhead. 7 The heavy overhead acid product had a petrolatum melting point of 138 F., a flash point of '465 F. and a fire point of 495 F.

The wax acid was tested for activity by the procedure used in Example I. The results showed that the tin oxides were readily removed from the surfac'e'of the tin.

Example ,III

' A C1s+ wax acid mixture, having a saponification number of 131, was obtained by oxidizing to complete conversion C34 to C55 microcrystal'line wax and extracting'thecrude acid product (saponification number of 208) with propane. The pure acid product contained an'average' of about twenty-eight carbon atoms per molecule.

The wax acid was tested for its activity by the procedure used in Example I. p The activity was good and lasted for fifty minutes. [Palm oil has an activityfor twenty minutes] f Example I V A C1a+ Wax acid mixture, having a'saponification number of 136,'was obtainedby oxidizingto complete conversion C34 to C55; microcrystalline wax, washing the reaction product with hydrochloric acid and water .(crude saponification number of 208) and distilling the washed The acid bottoms, which'contained an average.

product. of about twenty-seven carbon atoms per molecule, had a flash point of 515 F. f

The Wax acid was tested for its activity by the procedure used in Example I. The activity was good and lasted for thirty minutes.

Example V A Cra-lwax acid mixture was obtained by oxidizing to completeconversion C34 to C55 microcrystalline wax and extracting the crude acid product (saponification number of 208) with propane. The pure acid product contained an average of about twenty-eight carbon atoms per molecule. It was blended with an equal portion of a solvent refined bright stock oil.

The blended product was tested for its activity by the procedure used in Example I. The activity was fair and lasted for over twenty-five minutesJ Example VI The wax acid was tested for its activity by the procedure used in Example I. The activity was good and lasted for over sixty minutes.

I claim: In hot dip tinning operations wherein freshly-tinned steel plate is passed through a bath of a tinning oil maintained at an elevated temperature, the step of contacting the steel plate with a tinning oil comprising a mixture of high molecular weight microcrystalline wax acids produced by substantially complete oxidation of microcrystalline wax containing 34 to carbon atoms per molecule which is characterized by extreme water insolubility and by a saponification' number less than about 200 and which predominates in monocarboxylic acids having an apparent chain length exceeding eighteen carbon atoms per molecule.

References Cited in the file of this patent UNITED STATES PATENTS 

